Move interface.js to within khan-exercises

(No other changes)

Test Plan:
Loaded Addition 1.
Split out most exercise interface code

There is no scenario in which this is not a bitch to review. Sorry.

TODO: make local mode work again
TODO: get rid of utils/hints.js and merge with exercise-package/interface.js

Test Plan:
Did problems in Addition 1 locally and took hints and saw buttons update and lost leaves.